Ukraine's government has formally ended an Easter truce and is relaunching the “active phase” of an “anti-terrorist operation” to eliminate armed pro-Russian separatist groups in the east, First Deputy Prime Minister Vitaly Yarema said on Wednesday.

Yarema, who has a role in security matters, told reporters on the sidelines of a weekly cabinet meeting the new action was not yet underway, but added: “In the near future, appropriate measures will be taken and you will see results.”

Speaking a day after Vice President Joe Biden visited Kyiv, he added: “We have obtained the support of the United States, that they will not leave us alone with an aggressor. We hope that in the event of Russian aggression, this help will be more substantive.”

Earlier, Ukraine's acting president accused Russia of aiding “terrorists,” urging a crackdown on pro-Moscow separatists in the east on Tuesday that could hamper international efforts to defuse the crisis.

Since Russia signed up to the four-way accord in Geneva along with Ukraine and Kyiv's U.S. and EU allies, militants who deny taking orders from Moscow and whom Russia denies it controls, have if anything tightened their grip on about a dozen public buildings they seized over the past two weeks in eastern towns and cities.

Turchynov's call may not lead to much more action but it may fuel recriminations between Moscow and Kyiv about who is failing to honor the deal.

Washington has called on Moscow to live up to commitments made in Geneva last week to put more pressure on the pro-Russian protesters to vacate state buildings.

Blaming Kyiv for the tensions, Russian Foreign Minister Sergei Lavrov told his U.S. counterpart, John Kerry, in a phone call on Tuesday that authorities there must take "urgent steps" to implement the Geneva accord, a Russian Foreign Ministry statement said.

Speaking with with Ukraine's Prime Minister Yatsenyuk, Secretary of State John Kerry praised the important steps taken to de-escalate, in accordance with the Geneva agreement.

Many of the separatists demand referendums in Ukraine’s eastern regions similar to the poll last month in Crimea which precipitated the peninsula’s annexation by Russia.

Biden visit

Wrapping up a two-day visit to Kyiv, Vice President Biden called on Russia to live up to its recent agreement to resolve the crisis, adding that Ukraine is "in the struggle for its very future."

During a joint appearance Tuesday with Prime Minister Arseniy Yatsenyuk, Biden said it is time for Russia "to stop talking and start acting" on commitments it made to withdraw support for pro-Russian separatist forces in Ukraine’s east.

"We've heard a lot from Russian officials in the past few days. But now it's time for Russia to stop talking and start acting," he said. "We will not allow this to become an open-ended process. Time is short in which to make progress."

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The U.S. vice president said Ukraine "is and must remain one country." He said the United States would not let Ukraine "walk down this road alone... we will walk it with you."

Biden also said the U.S. would provide an additional $50 million for the Ukrainian government, including $11 million to help fund the May 25 presidential election.

A White House statement Tuesday said the U.S. will provide Kyiv with $8 million in non-lethal military aid, including communications equipment and vehicles.

U.S. boosting regional troop presence

The U.S. is sending about 600 soldiers to Poland and the three Baltic states for infantry exercises, the Pentagon said on Tuesday, one of its highest-profile steps yet to reassure NATO allies after Russia's seizure of Crimea.

Groups of 150 soldiers will be sent to Poland, Estonia, Lithuania and Latvia in rounds of month-long exercises in each country in what the Pentagon calls a “persistent rotational presence” that stops short of permanently basing troops.

Rear Admiral John Kirby, a top Defense Department spokesman, said the deployments sent a strong message to NATO allies about U.S. commitments to the alliance following events in Ukraine.

“If there's a message to Moscow, it is the same exact message that we take our obligations very, very seriously on the continent of Europe,” Kirby said, leaving open the possibility that the drills, set to last through 2014, could continue into 2015.

The United States and NATO have made clear they will not intervene militarily in Ukraine, which is not a NATO member.

They say they are instead focusing on temporarily boosting their presence in eastern Europe in a drive to reassure allies, such as former Soviet republics Estonia, Latvia and Lithuania that NATO would protect them if they ever faced Russian aggression.

With their own Russian-speaking minorities, the three Baltic countries have been increasingly worried that Russia's annexation of Crimea, partly on ethnic grounds, could herald destabilization in their own region by Moscow.